This invention relates to optical gyroscopes in general and to a fiber optic gyro wherein a digitally synthesized serrodyne waveform is applied to a single phase-modulator in a closed-loop fiber optic gyroscope.
In fiber optic gyroscopes there exists an analogy to traditional electromechanical rate gyros in that either type may be operated "open-loop" or "closed-loop". The closed-loop configuration generally is associated with higher levels of precision than the open-loop type. In an open-loop gyro the basic device output is the direct measure of gyro input rate. In a closed-loop gyro the output is constantly maintained at null by means of an external feedback mechanism which acts to cancel the inertially sensed rotation. In the case of electromechanical gyros this feedback consists of applying a precision torque to the gyro's spinning rotor to maintain the alignment with the gyro case, the current needed to apply the correct torque thus becoming a measure of gyro input rotation. In the case of fiber optic gyros, the appropriate photodetector output current component is continuously maintained at null by the action of a differential phase shift transducer which exactly cancels the phase shift induced by the applied input rate (the "Sagnac" shift) to the two counterpropagation light beams within the fiber. The frequency of the signal voltage to the tranducer becomes the measure of gyro input rate.
In order to generate the differential phase shift, two basic approaches have been employed in the past. One involves the use of acousto-optic frequency shifters (e.g. Bragg cells) which directly change the optical carrier frequency of an input light beam by the amount of the applied signal voltage frequency. Another approach to phase-shifting a light beam is to employ a phase shifter which can vary or modulate the optical path length within itself by application of a signal voltage to it. These phase-shifters (which are several) may be fabricated so as to be an integral part of the fiber coil or formed with vibrating mirrors, or made in integrated optic form or formed by deposition of optically active material on optical fiber. To generate a differential beam phase shift equal and opposite to the Sagnac-induced (i.e. rate induced) phase shift, a special electrical periodic signal in the shape of a sawtooth (a serrodyne waveform) is applied to the phase shifter. The repetition rate of this periodic waveform then becomes the gyro output, along with an additional output to specify the direction of input rate (polarity).
A problem that arises with such a serrodyne modulator when gyro input rates are low is that it becomes difficult to generate the serrodyne voltage waveform. Another problem that arises is the gyro's ability to track rapid variations in input rates is limited. Both difficulties ultimately result in inaccurate gyro scale factor performance.
To overcome the problem of low input rate operation of a serrodyne-modulated fiber optic gyro, several approaches have been implemented in the past. One such approach is to generate variable voltage steps rather than a continuous voltage ramp. One disadvantage is the need to gate or switch out transient pulses. Such gating is a gyro output error source and complicates the electronics circuitry. Another disadvantage is a relatively complex electronics processor that is needed to generate the variable-height phase steps. Another disadvantage is that this method may impose a maximum rate magnitude beyond which the gyro output is not uniquely associated with its input or a limitation on the fiber coil diameter resulting in poor sensitivity.
Another approach to avoid the need to generate serrodyne waveforms at low gyro input rates is to apply a relatively high bias frequency serrodyne voltage to the phase modulator. A disadvantage is one earlier mentioned that in regard to maximum input rate limitation. In addition, this approach generates a large difference in interbeam optical carrier frequency at low gyro input rates resulting in poor gyro bias drift stability.
In a co-pending serrodyne system application Ser. No. 302,484 "Fiber Optic Gyroscope Plurality Modulators", assigned to the same assignee as the present invention, there is shown a serrodyne gyroscope using two serrodyne generators and two phase modulators, each modulator operating at fairly high frequencies irrespective of input rate, magnitude or direction. The two phase modulators operate as conventional baseband serrodyne modulators (pure frequency shifting) about a quiescent finite frequency in push-pull. The gyro output becomes the frequency difference between the signals applied to the two modulators. When two modulators are used a differential phase shifter mechanism evolves whereby the two light beams are phase-shifted with respect to each other in proportion to the difference in frequency of the two serrodyne waveforms as respectively applied to the two modulators. Thus any arbitrarily small phase shift (including zero) can be generated without requiring low-frequency voltage waveforms for either modulator by itself.
Another co-pending serrodyne system application Ser. No. 302,157 "Fiber Optic Gyroscope Combined Signal Phase Difference Control", assigned to the same assignee as the present invention, describes a synthetic single serrodyne modulator for fiber optic gyroscope which retains the advantages of baseband serrodyne (pure frequency shifting) and requires only one phase modulator. In synthetic serrodyne, the two serrodyne generators operating around a quiescent frequency are summed electronically along with the bias modulation and then applied to the single phase modulator. This co-pending case using a single phase modulator results in a more simple, more cost effective design and offers improvement in random rate noise, bias stability and a scale factor which is much improved over open-loop versions. The teaching of these two co-pending applications (to the extent necessary) is incorporated by reference herein.
In the present invention a digitally synthesized serrodyne waveform is applied to a single phase modulator in a closed-loop fiber optic gyroscope. The one phase modulator is operated as a baseband serrodyne frequency shifter. The applied serrodyne signal is synthesized from the digital summation of two binary words which are digital-to-analog converted and amplified. Only one op-amp is required. Reset time is minimized.